Gas odorization has become a part of daily life especially with the widespread utilization of natural gas in households and industry, and measures to be taken in terms of security are of vital importance. Since natural gas which is not supplied for utilization (>95% methane gas) is odorless, it cannot be sensed by the users in case of any leakage. In order to enable that any possible natural gas leakages are detected before its concentration in air reaches to the lower flammability limit, mercaptan compounds have been started to be added into natural gas since 1940s. LPG is a byproduct of natural gas and petroleum refining processes and it is supplied from the points where the said refining is performed. The supplied LPG may comprise sulphur containing compounds, in various types and proportions, according to the source of production. While sulphurous compounds may be contained in LPG obtained from refining of crude petroleum in various types and higher amounts depending on the refining process, they are generally lower in LPG originating from natural gas. Based on that, LPG presents a characteristic odor profile due to sulphur compounds contained. Depending on the amount of sulphurous compounds in LPG, it might not be necessary to additionally odorize it in certain cases. On the other hand, LPG which contains lower proportions of sulphur compounds is subjected to odorization. In the selection of the odorants used in odorizing, a criterion is applied which is based on the fact that LPG odor, in terms of its odor nature, is unpleasant and distinctive from odors which can be easily encountered in daily life. Currently, among the main odorizing chemicals widely used in the LPG sector in the world, sulphurous compounds such as methyl mercaptan, ethyl mercaptan, t-butyl mercaptan, n-propyl mercaptan, isopropyl mercaptan or tetrahydrothiophene, dimethyl sulfide and diethyl sulfide are included. Apart from the nature of the odor, other important criteria used in the selection of the said odorants are intensity of the odor, and the physical and chemical characteristics of the odorants. LPG is a fuel used in various areas, which is used in heating, cooking, illumination, as vehicle fuel and as propellant in perfumes. Most of these utilization areas necessitates that LPG that is procured to the consumer is odorized.
An odorant commonly used in LPG sector is Ethyl Mercaptan (EM), which contains sulphur at a level of 52% in its molecular structure. In order to comply with the condition of TS EN 589 standard which stipulates that ‘The odor of the gas should be specific (distinctive and unpleasant) and its odor should be detectable when its concentration in air is less than 20% of its lower flammability limit’, the amount of EM dosed into LPG is approximately 20 ppm depending on the odor description threshold and volatility of EM. The lower and upper explosion limits of Liquefied Petroleum Gas-air mixture are 1.55% and 9.6%, respectively. This EM sulphurous compound of 20 ppm added additionally in LPG increases the sulphur content of LPG by approximately 10 ppm. As a result of this EM addition, the sulphur content in 1 ton of LPG is increased by 10 gr. Considering 3.5 million tons of LPG market, this value corresponds to approximately 35 tons of elemental sulphur content. As a result of conversion of 35 tons of sulphur into SO2 gases in engine and combustion systems, SO2 emissions increase.
In automotive sector, for purposes of converting environmentally hazardous exhaust gases that are released during fuel consumption, into less hazardous gases through oxidation, catalytic converters are used in vehicles. Due to the susceptibility of the catalyst substances (Pt—Rh/CeO2—Al2O3) used in catalytic converters to sulphur, exhaust gases with high sulphur content increase the amount of hazardous gas released into the atmosphere by negatively affecting oxidation performances of the catalytic converters. Such effect of sulphur on catalyst substances is not permanent, and with a decrease in the sulphur content of the fuel used, the negative effect on the oxidation performance disappears. In this respect, decreasing the sulphur content of LPG used as auto-gas will not only result in a decrease in SO2 emissions, but also in the emission amounts of all hazardous exhaust gases emitted into the environment during auto-gas consumption.
Liquefied Petroleum Gas means liquid gas which can be converted into liquid phase generally at 20° C. and under 3.5 Bar pressure. Basically, it consists of n-propane, propylene, n-butane and butylene. With a narrower description, it is liquid gas consisting of mixtures of n-propane and n-butane. This mixture may contain low amounts of unsaturated hydrocarbons and/or branched hydrocarbons such as propylene, isobutane, 1-butylen, cis-2-butylene, trans-2-butylene or isobutylene.
Liquefied Petroleum Gas is generally transported without going through any odorizing process. Odorizing process is performed at the storage facilities. During the odorizing process, the storage tank is supported with nitrogen against explosion risk. According to TS TSE/TS 8038 Standard, the amount of odorant required to be added into Liquefied Petroleum Gas is calculated as follows: when the concentration of the gas in air is equal to 20% of the lower explosion limit, in order to allow the odor to reach warning level, the required odorant concentration (C) in Liquefied Petroleum Gas can be roughly calculated with the following formula, in mg/m3: C=(K.100)/(0.2.APS)
Wherein, K defines the odor sensing threshold. K values for certain odorants are as follows:
OdorantK value, mg/m3Tetrahydrothiophene0.075Mercaptans0.04-0.09Dimethyl sulphur0.28
Sulphur compounds are often used in liquefied petroleum gas compositions. Sulphurous compounds are hazardous to human health, environment and machine parts. When using odorants containing sulphurous compounds, and such odorants are used with LPG, emissions arising from consumption of LPG as bottled gas and auto-gas have adverse effects on humans and other living creatures in terms of below mentioned aspects. With the utilization of sulphur-free odorant, the said adverse effects will be eliminated.
Hazards to Humans and Other Living Creatures
Compounds containing sulphur, when exposed to high amounts thereof, may cause damage on the cell structure of living creatures. Thioltransference, which catalyzes substitution reaction with glutathione and shows high degree of activity in the organs and tissues, is affected in the first order by the dialkyl disulfide toxicity (Lillig and Holmgren, 2007). The reaction mechanism is quite important because it is related with the free radical medium with excessive and high reactivity, which may initiate the redox cycle in tissue macromolecules or in the sites they form (FIG. 1).
The mechanism of free radical formation from dialkyl disulfide and the reaction steps of redox cycle are shown below (Munday and Manns, 1994). The first product of thiol transference substitution reaction is an alkyl mercaptan (1); after being ionized, undergoes a single electron oxidation (2) and free radical intermediate phase occurs. This intermediate product is toxic and it is a constant hydroxyl radical producer and other reactive oxygen examples can maintain the redox cycle (3, 4, 5, 6) and they cause oxidative stress and tissue damage in the sites they form.2 GSH+RSSRGSSG+2 RSH  (1)RSHRS—+H+  (2)(Hb)Fe3O2•—+RS—+2H+→(Hb)Fe3+RS•+H2O2  (3)RS•+RS—(RSSR)•−  (4)(RSSR)•—+O2→RSSR+O2•—  (5)RSH+O2•—+—H+→RS•+H2O2  (6)
Long chain lengths in a molecule decrease the radical stability, thereby reducing oxidation rate (Munday, 1989). Furthermore, the reactivity and toxicity of alkyl disulfides is reduced as follows due to the effect of steric factors on the thioltransference activity: n>sec>tert. According to this information DMDS is the most reactive member of the homologous sequence in terms of chain length and branching.
Additionally, Fe and its oxides cause damages to the storage tanks by showing the following reactions with H2S:Fe+H2S→FeS+H2  (7)Fe2O3+3H2S→2FeS+3H2O+S  (8)2Fe(OH)3+3H2S→2FeS+6H2O+S  (9)Fe(OH)3+3H2S→Fe2S3+6H2O  (10)Fe3O4+4H2S→3FeS+4H2O+S  (11)Acid Rains
Combustion of sulphurous fossil fuels is the main source of SOx. Formation of SOx results from SO2 arising out of combustion, in a proportion between 97% and 99%. The remaining part is mostly sulphur trioxide (SO3). This compound available in the atmospheric water vapor rapidly transforms into H2SO4. When in sufficient concentrations, SO2 and H2SO4 are hazardous to respiratory system. Besides, SO2 is also toxic to plants (U.S. EPA, 1999).
Catalytic Converter Intoxication
Sulphur intoxication is a complicated event which alters the structural, morphological and electronic characteristics of the catalyzer (Rodriguez & Hrbek 1999). Sulphur negatively affects the activity and oxygen storage capacity of the catalyst (Boaro et al. 2001, Yu & Shaw, 1998). The existence of sulphur may cause formation of new inactive compounds on the surface of the catalyst. Furthermore, it may also cause structural changes in the catalyst (Yu & Shaw, 1998).
Depending on the temperature and partial pressure of oxygen, sulphur contained in the exhaust gas may be converted into sulfate, sulfide or oxy-sulfides by the catalyst (Karjalainen et al. 2005). At temperatures below 300° C., these oxides are adsorbed by the active surfaces on the surface of the catalyst and reduce the active surface, so the efficiency of the catalyst decreases. Under reduction conditions, sulphur forms H2S and intoxicates metal surfaces, and negatively affects the oxidation of hydrocarbons (Rabinowitz et al., 2001). In case of a rich mixture of SO2, sulphur deactivation is more important in the presence of NOx, and even at 1000° C. very stable sulfates may form, without being attacked by reducing agents, especially in the absence of water (Fridell et al. 2001, Mahzoul et al. 2001).